Amid a sour economy and higher airfares, the U.S. airlines' trade group Tuesday forecast the biggest decline in summer air travel since the summer after the Sept. 11 attacks.

The Air Transport Association, which represents the 13 biggest U.S. passenger carriers, forecast 211.5 million passengers will fly on U.S. carriers June 1 through Aug. 31, a nearly 1.3% drop from last summer.

Passenger traffic on flights within the USA will be down almost 2%, while the number of passengers on international flights will be up slightly, the ATA predicted. The forecast is based largely on summer flight schedules and trends in fares and passenger traffic.

If the forecast holds true, this summer will see the first falloff in summer passenger traffic since summer 2006, when the arrests of London terrorists plotting to bomb airplanes prompted tighter security restrictions and frightened off some travelers. The number of passengers flying U.S. carriers that summer dropped 0.7%, according to government data.

After the September 2001 attacks, passenger traffic the following summer was off 8.5%.

ATA officials blame the trend this year on the U.S. economic slowdown as well as higher ticket prices that airlines are charging to try to recoup record fuel costs. Fuel is an airline's single-biggest expense, and the price of jet fuel has jumped 63% since last May.

ATA chief James May said Tuesday that jet-fuel prices are going into "absolutely uncharted territory."

ATA chief economist John Heimlich, author of the summer forecast, said that he believes there's evidence that higher fares are putting a dent in passenger demand. Since Christmas, fuel prices have forced seven small airlines — several of them discounters — out of business. Another low-cost carrier, Denver-based Frontier Airlines, recently sought Chapter 11 bankruptcy-court protection.

"The carriers that went out of business had the most price-sensitive clientele," Heimlich said. "Higher energy prices affect lower-income customers the most."

U.S. carriers have announced fare increases 11 times since late December, but competition has kept them from sticking fully on every route. The latest fare increase came Friday, when American, United, Delta, Continental, Northwest and US Airways added a $20 fuel surcharge to the price of their tickets.

Yet on Tuesday, discounter AirTran Airways launched a 72-hour sale on flights to all of its destinations for travel through early November.

"As we take fare increases and add fuel surcharges, consumers are not paying them," said AirTran spokesman Tad Hutcheson. "There is some sticker shock out there. People wait for a sale."

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